Two different types of image sensors can be realized in CMOS technology: passive and active pixel sensors (APS). The difference between these two types is that a passive pixel does not perform the signal amplification performed by an active pixel. A passive pixel sensor may simply comprise a photodiode (MOS or p-n junction diode) with a transistor that passes photoelectrically generated signal charge to an amplifier outside the pixel array. In an active pixel, the integrated charge is amplified by a source follower transistor, for example, and then transported in the channel of a reset transistor.
With regard to both passive and active pixel sensors, it has been determined that excessive current leakage from the photosensitive (e.g., photodiode) regions is a significant cause of white pixel problems. It is a problem that, despite many previously attempted solutions, there are still large numbers of white pixels in CMOS image sensors. In a CMOS image sensor, current flow from the photodiode to ground is used to measure the intensity of incident light. Any additional source of leakage current will adversely affect the performance of the CMOS image sensor. That is, defective pixels will appear to be in the presence of a bright light source even when the pixel is exposed to darkness. Such pixels are called “white pixels” since these locations in the CMOS image pixel array always appear to be “white” regardless of the actual incident image.